Growth of cadmium oxide single crystals



Aug. 10, 1965 T. c. M AvoY ETAL 3,199,961

GROWTH OF CADMIUM OXIDE SINGLE CRYSTALS Filed May 23, 1962 POLYCRYSTALLINE C0 PLACEB lN Thus AREA R I000 Q o son-g1,

DIRECTION OF R CARRIER Gms 600 E I POSITION OF'G'ROWTH 400 ruse:

o a 4 6 8 IO la l4 16 I8 210 01s rA we: 41. o/va MUFFLE e l/vcwss INVENTORS Tao/was 6. M4c4vor Alva JAMES A. lVARL-gJk.

United States Patent 3,199,961 GROWTH OF CADMIUM ()XEDE SINGLE CRYSTALS Thomas C. MacAvoy and James A. Marley, .lr., Corning,

N .Y., assignors to Corning Glass Works, Corning, N.Y.,

a corporation of New York Filed May 23, 1962, Ser. No. 197,091 3 Claims. (or. 23-294) This application is a continuation-in-part of our copending application Serial No. 103,362, filed April 17, 1961, which application relates to the growth of large single crystals of stannic oxide.

This invention relates to methods for growing crystals and more particularly to a method for growing large single crystals of high purity from polycrystalline materials.

Experimentation subsequent to the filing of the cited application has shown the universality of an advantage observed in conjunction with the growth of stannic oxide crystals according to the method disclosed therein. In that application it was observed that unusually pure stannic oxide single crystals can be grown by a vapor deposition process wherein vapors produced by the sub limation of pure polycrystalline stannic oxide are transported by means of a helium carrier gas to a growth site, where they are deposited in the form of stannic oxide single crystals. Although it was not at that time known whether the advantages found in the use of helium as a carrier gas would be found in processes for growing single crystals of other materials by vapor deposition, it was believed that such might be the case, and it was hypothesized that the advantages might result from the high thermal conductivity of helium and its consequent ability to absorb and transport heat of crystallization away from the growing crystals.

It has since been determined that the use of helium as a carrier gas in the growth of single crystals by vapor deposition improves the quality of all single crystals so grown over that obtained with gases having lower thermal conductivities. It has been found that the improved quality is particularly apparent in the growth of single crystals of substances which themselves have low thermal conductivities, such as metallic oxides, where heat produced by crystallization at the surface of the growing crystal is particularly slow to dissipate through the crystals.

Accordingly, it is an object of this invention to provide an improved process for the vapor deposition of single crystals.

It is a further and more particular object of the invention to provide a method for the vapor deposition of large single crystals of cadmium oxide.

Although the invention will be hereinafter described as utilized in the production of single crystals of cadmium oxide, it will be understood that such description is given only by way of example and that the present invention, which resides in the use of helium as a carrier gas in the vapor deposition of single crystals, is applicable in the vapor deposition of single crystals of all materials. For example, but not by way of limitation, the process of the present invention will be found particularly advantageous in the vapor deposition of single crystals of the oxides of barium, zinc, magnesium, aluminum, indium, and beryllium.

The invention is illustrated in the accompanying drawing, in which:

FIG. 1 is a schematic representation, partly in section, of a crystal-growing furnace used in the process of the present invention, and

FIG. 2 is a curve representing typical temperature 3,199,951 Patented Aug. 10, 1965 ice variations which may be maintained along the length of the furnace represented in FIG. 1.

Pure polycrystalline cadmium oxide 15 was placed, as illustrated in FIG. 1, in growth furnace 10, which comprises a hollow tube, or muffle, 11, around which are wound three independently controlled heating coils 12. Muifle 11 is approximately 36" long and 1%" in inner diameter, although these dimensions may be varied. The temperature gradient along the interval of approximately 20" which extends from cadmium oxide powder 15 through growth site 18 may be adjusted to conform to a typical curve, illustrated in FIG. 2, by independently regulating the power inputs to coils 12. The carrier gas is introduced through inlet 16, and additional gases may be introduced through inlet 17 for purposes which will be hereinafter stated. The gases introduced through inlets 16 and 17 travel over growth site 18, which is a removable tube closed at its end nearer the cadmium oxide powder, and emerge from outlet 19.

The vaporization of cadmium oxide is believed to take place according to the equation:

2CdO= 2Cd+0 Accordingly, the process of the present invention is believed to operate by the generation of cadmium vapor and its subsequent reoxidation and deposition in the form of cadmium oxide single crystals.

Approximately 50 grams of cadmium oxide powder 15 are placed as shown in the furnace illustrated in FIG. 1. The temperature within muffle 11 at this location is maintained high enough to permit vaporization of the cadmium oxide, or somewhere above 900 C., for example, approximately 1250 C., as indicated in FIG. 2. The temperature gradient along the remainder of muflle 11 is adjusted as indicated in FIG. 2.

The carrier gas transports the cadmium vapor and the oxygen which are formed by the vaporization of cadmium oxide along muflle 11 to the area of lower temperature found along growth site 13. Here the lower temperatures cause progressively increasing supersatuation of the vapor, which results in condensation and crystallization of cadmium oxide along the growth site.

Mufile 11 is preferably made of platinum and growth site is is preferably made of magnesium oxide.

When helium is used as the carrier gas, the qualityof the resultant cadmium oxide crystals is found to be far superior to that of crystals produced using other carrier gases. When helium is utilized as the carrier gas, large single crystals are formed which are completely free of voids and imperfections, while, when the carrier gas is argon, which has a thermal conductivity approximately one-tenth that of helium, irregular crystal surfaces and numerous voids are found. The only other carrier gas having a thermal conductivity of the same order of magnitude as helium is hydrogen. However, hydrogen cannot be used in growing oxide crystals since it would combine with the free oxygen.

Tube 17 is provided for the admission of additional oxygen between cadmium oxide powder 15 and growth site 18, in order to insure complete reoxidation of the cadmium vapor to form pure cadmium oxide crystals.

Cubic single crystals of cadmium oxide have been grown according to the present method up to 1 cm. on an edge.

Although the advantages found in the use of helium as a carrier gas are particularly evident in the growth of metallic oxide crystals, improved crystals of other materials may be so produced. For example, it is advantageous to use helium carrier gases in producing by vapor deposition single crystals of the semi-conductor convert said cadmium oxide toa vapor, passing a carrier gas comprising helium over said quantity of cadmium oxide, carrying said vapor in said carrier gas to a growth site having a temperature within the range 800 C.-1260 0, whereby to deposit said vapor on said growth site in the form of at least one single crystal.

2. The method of producing single crystals of cadmium oxide which comprises the steps of heating a quantity of cadmium oxide to its sublimation temperature to convert said cadmium oxide to a vapor, passing over said quantity of cadmium oxide a carrier gas comprising helium, carrying said vapor in said carrier gas to a growth site having a temperature within the range 800 0-1260 C., and-injecting oxygen into said carrier gas in the vicinity of said growth site, whereby to deposit said vapor on said growth site in the form of at least one single crystal.

3. The method according to claim 1 in which said growth site comprises an area having a temperature varying between approximately1260 C. at a point near said quantity of cadmium oxide and approximately 800 C. at a point farther from saidquantity.

References Cited by the Examiner UNITED STATES PATENTS Re. 23,556 9/52 Mochel l17-106 1,161,885 11/15 Rigg 23-183 1,161,886 11/15 Rigg 23183 1,964,322 6/34 Hyde. 1,973,590 ll/34 Weaton et al. 2,474,645 10/ 39 V Barter 117106 OTHER REFERENCES Semiconductors by Hannay, pages 134 to 141, Feb. 27, 1959, Reinhold Pub.'Corp.

Lawson et al.: Preparation of Single Crystals.1958, pages 21 to 25, 89 to 92, and 204 to 209, Butterworth (London), QD 931 L3 195821.

Mellor: Comprehensive Treatise oflnorganic and Theoretical Chemistry. 4

NORMAN YUDKOFF, Primar Examiner. GEORGE D. MITCHELL, Examiner. 

1. THE METHOD OF PRODUCING SINGLE CYRSTALS OF CADMIUM OXIDE WHICH COMPRISES THE STEPS OF HEATING A QUANTITY OF CADMIUM OXIDE TO ITS SUBLIMATION TEMPERATURE TO CONVERT SAID CADMIUM OXIDE TO A VAPOR, PASSING A CARRIER GAS COMPRISING HELIUM OVER SAID QUANTITY OF CADMIUM OXIDE, CARRYING SAID VAPOR IN SAID CARRIER GAS TO A GROWTH SITE HAVING A TEMPERATURE WITHIN THE RANGE 800*C.-1260* C., WHEREBY TO DEPOSIT SAID VAPOR ON SAID GROWTH SITE IN THE FORM OF AT LEAST ONE SINGLE CRYSTAL. 